1. Field of the Invention
The present invention relates to technology for joining different kinds of metal members, such as joining a magnetic material and a slide member, and relates to a sound welded structure in the laser junction technology and to a welding method.
2. Description of the Related Art
Referring, for example, to FIG. 1, a drive unit of a magnetically driven actuator is constituted by the combination of a slide member 91 having excellent wear resistance and a magnetic member 92 having excellent magnetic properties. A variety of junction methods can be employed for this combination.
A representative junction method may be a mechanical caulking method which is shown in FIG. 2. That is, a recessed portion 915 is formed in advance in the junction portion of the slide member 91, the magnetic member 92 is fitted thereon and is ironed from the outer side, so that the inside of the magnetic member 92 bites into the recessed portion 95 enabling the slide member 91 and the magnetic member 92 to be joined together. According to this method, however, the caulking strength is so small that the reliability of junction is poor.
There can be exemplified a so-called brazing method. According to this junction method, however, the member as a whole must be heated, causing the wear-resistant member to lose its hardness. Besides, the dimensional precision is deteriorated due to thermal distortion causing disadvantage even from the standpoint of machining cost.
As shown in FIG. 3, furthermore, there has been proposed a junction method based upon the laser-welding by irradiating a portion where the slide member 91 and the magnetic member 92 are overlapped one upon the other with a laser beam 8. According to this method which is based upon the local heating, the dimensional precision is not lost by heating, hardness is not lost, a large junction strength is obtained and a high machining rate is accomplished. It can, therefore, be said that the laser-welding could become a junction method featuring excellent productivity and low cost.
However, the following problems are involved in the welded structures obtained by the above-mentioned conventional laser-welding method.
Referring, for example, to the case of laser-welding a magnetic member and a slide member together, a ferrite type stainless steel or a nickel-iron type material is usually used as the magnetic member, and a high-carbon steel such as martensite type stainless steel having excellent hardenability is usually used as the slide member.
The ferrite type stainless steel or the nickel-iron type material has a relatively favorable weldability, but the high-carbon steel such as the martensite type stainless steel has poor weldability and is often cracked upon welding. When these different kinds of metal members are laser-welded together, therefore, cracks are very likely to develop in the melt-solidified portions being affected by the high-carbon steel that has poor weldability.
Concretely speaking, cracks 99 often develop in the melt-solidified portions 2 as shown in FIGS. 4 and 5.
To prevent the occurrence of cracks in the welded portions, the following methods may be employed; i.e.,
(a) a method wherein the depth of melt of the slide member is decreased, and a volume ratio of a melt of the magnetic material is increased, in order to decrease the amount of the carbon component which causes cracks in the molten portion; PA1 (b) a method wherein the magnetic material is replaced by a high-nickel steel such as permalloy to improve the weldability, and the carbon component in the slide member is lowered; or PA1 (c) a method of employing a welded joint which permits tensile stress to build up little in the melt-solidified portion. PA1 a first member and a second member of metal materials of different kinds which are overlapped one upon the other, and a melt-solidified portion formed by the irradiation with a laser beam and arriving at said second member from the surface of said first member penetrating through said first member; wherein PA1 said melt-solidified portion has nearly an inverted triangular shape in cross section with its width becoming gradually narrow from the surface of said first member toward the inside thereof, and has a low-hardness layer on the front surface side of said melt-solidified portion and a high-hardness layer on the inside thereof; and PA1 said high-hardness layer has a structure in which first layers and second layers having a hardness smaller than that of said first layers are alternatingly laminated in a direction in which the welding proceeds. PA1 said laser beam is so projected that said first member and said second member are melted by the laser beam of a first pulse to thereby form a first molten portion, projection of the laser beam of a second pulse is started before the growth of solidification of said first molten portion reaches the distance of motion of the laser beam axis thereby to form a second molten portion in a manner that it partly overlaps said first molten portion and, then, the third and subsequent pulses of the laser beam are successively projected being turned on and off maintaining a predetermined interval, in order to form a melt-solidified portion having a low-hardness layer formed on the front surface side and a high-hardness layer on the inside, the high-hardness layer being formed by alternatingly laminating first layers and second layers having a hardness smaller than that of said first layers in a direction in which the welding proceeds.
However, the above-mentioned methods (a) to (c) permit new problems to take place, and are not capable of solving the problems.
That is, according to the method (a), the depth of the molten portion and the hardness of the molten portion decrease, the welded strength loses stability, and reliability is lost. The method (b) causes the magnetic properties to decrease and drives up the cost of the materials. The method (c) impairs the degree of freedom in designing the product, causing disadvantage in the structure and in the cost of the product.
These problems are not limited to the case of laser-welding the magnetic member and the slide member together but could similarly occur in laser-welding various different kinds of metal materials.
The present invention was accomplished in view of the above-mentioned problems inherent in the prior art, and provides a laser-welded structure having sound melt-solidified portions without cracks and a laser-welding method therefor.